Optically active baclofen [4-amino-3-(4-chlorophenyl) butanoic acid] is a raw material of medicine, or an intermediate thereof (see, for example, U.S. Pat. No. 7,354,954).
As a process for producing optically active 4-amino-3-(4-chlorophenyl)butanoic acid, known is, for example, a process described in US2009/0137819, or a process described in Journal of the Chemical Society, Perkin Transactions 2, 1997, pp. 763-768.
In the process described in US2009/0137819, a crystal of optically active 4-amino-3-(4-chlorophenyl)butanoic acid is yielded by a method of adding an aqueous solution of sodium hydroxide into an aqueous solution of a hydrochloride salt of optically active 4-amino-3-(4-chlorophenyl)butanoic acid to attain neutralizing crystallization, or in the process of Journal of the Chemical Society, Perkin Transactions 2, 1997, pp. 763-768, the same salt is yielded by a method of adding diluted hydrochloric acid to an aqueous sodium hydroxide solution of optically active 4-amino-3-(4-chlorophenyl) butanoic acid to attain neutralizing crystallization. The journal also states that the crystal is further heated in methanol. However, these literatures never disclose that polytype crystals of optically active 4-amino-3-(4-chlorophenyl)butanoic acid exist.